1. Field of the Invention
The invention relates generally to an optical component with a low reflectance for ultraviolet light in a wavelength range between approx. 150 nm and approx. 250 nm at large angles of incidence, in particular between approx. 70° and approx. 80°.
2. Description of the Related Art
In many areas the need is increasing for powerful optical components with a low reflectance and high transparency or transmission for ultraviolet light in a wavelength range between approx. 150 nm and approx. 250 nm. Light from this wavelength range is used for example in microlithographic exposure systems for the production of highly integrated semiconductor components with the aid of wafer steppers or wafer scanners. In the process via an illumination system a light source illuminates a mask (reticle), the image of which is reproduced with the aid of a projection system onto a photoresist coated semiconductor wafer. As it is a known fact that the miniaturization achievable with this process increases the shorter the wavelength λ of the light used, in the most modem devices wavelengths from the deep ultraviolet range (Deep Ultraviolet, DUV) are used. Light sources for this are KrF excimer lasers with a working wavelength of λ=248 nm and ArF excimer lasers with a working wavelength of approx. λ=193 nm. These lasers generate linear polarized light, which, in the case of a diagonal incidence on a surface of an optical component, occurs either as s-polarized or p-polarized light, according to the individual surface orientation.
As is known the surfaces of transparent optical components are coated with so-called antireflection layers or antireflection layers (AR layers) to increase their transparency for light. Usually, in the process, multilayer systems consisting of several stacked layers of dielectric materials with various refractive indexes are used, in which layers of a high refractive material and layers of a relatively low refractive material are usually stacked alternately on top of each other.
Whereas for an effective reduction of reflection in the case of a vertical incidence of light a few layers can suffice if suitable layer materials are selected, experience shows that the number of layers required increases the bigger the angle of incidence Θ, i.e. the angle between the direction of the incidence of light and the surface normal. This effect is shown for example in EP 0 855 604, in which the antireflection layers for UV light in the wavelength range between 150 nm and 250 nm at large angles of incidence between 70° and 80° are revealed. The multilayer systems proposed there are characterized in that the optical thickness of the layers of high refractive material is always the same and the optical thickness of the intermediate layers of low refractive materials is always the same, so that a periodic layer sequence results. Examples are shown for p-polarized light with a wavelength of λ=193 nm, according to which in order to minimize the residual reflection to values of below approx. 0.5% at an angle of incidence of Θ=72° seven layers are required, at an angle of incidence of Θ=74° nine layers are required and at an angle of incidence of 76° even eleven layers are required. In the case of p-polarized light with a wavelength of λ=248 nm two additional layers each are required for the corresponding angles of incidence.
The practical use of optical components with antireflection multilayers is frequently influenced by the fact that these types of multilayer systems only show limited resistance when subjected to intensive high-energy UV radiation. As a result, the problem of the lacking laser resistance is pushed all the more to the fore the greater the energy density of the incident light. High energy densities of laser light occur for example in the field of devices for narrowing the bandwidth of excimer lasers. In U.S. Pat. No. 5,978,409 such a device is exemplarily shown, in which an arrangement of three or four prisms is provided to widen a laser beam before incidence on an echelle grate, on the hypotenuse surfaces of which the laser light always is incident with a large angle of incidence. In the case of an optimal configuration with regard to the achievable beam widening three prisms are provided, on the hypotenuse surfaces of which the UV light always impacts with angles of incidence of approx. Θ=74°. As for this configuration in the case of a wavelength of 193 nm, no sufficient laser resistant antireflection layer is available, uncoated prisms would have to be used, which would however lead to overall losses of more than 40% due to reflection in the case of the available substrate materials (CaF2 or synthetic quartz) and a double passage through the prisms. Therefore, as an alternative, an embodiment with four prisms is proposed, on the hypotenuse surfaces of which the laser light is incident with smaller angles of incidence between approx. 67° and approx. 71°. To reduce the reflection a single layer of Al2O3 is always applied to the surfaces, which has sufficient laser resistance and is also intended to lead to a sufficient reduction in reflection. However, the residual reflection can not be reduced below approx. 3% via such a single layer for angles of incidence of approx. 74°.
It is an object of the invention to provide an antireflection coating for optical components, which allows an effective antireflection coating for ultraviolet light in a wavelength range between approx. 150 nm and approx. 250 nm at large angles of incidence in the range of approx. 70° to approx. 80° and is characterized by high laser resistance.